Buick 2006 LaCrosse Owner's Manual page 279

You can run the engine to keep warm, but be careful.
CAUTION:
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Snow can trap exhaust gases under your
vehicle. This can cause deadly CO (carbon
monoxide) gas to get inside. CO could
overcome you and kill you. You cannot see it
or smell it, so you might not know it is in your
vehicle. Clear away snow from around the
base of your vehicle, especially any that is
blocking your exhaust pipe. And check around
again from time to time to be sure snow does
not collect there.
Open a window just a little on the side of the
vehicle that is away from the wind. This will
help keep CO out.
Run your engine only as long as you must. This saves
fuel. When you run the engine, make it go a little faster
than just idle. That is, push the accelerator slightly.
This uses less fuel for the heat that you get and it keeps
the battery charged. You will need a well-charged battery
to restart the vehicle, and possibly for signaling later on
with your headlamps. Let the heater run for a while.
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